FIG. 1 is a perspective view of an engine room 101 of a gasoline engine-driven vehicle (hereinafter, simply referred to as a vehicle) 100. The engine room 101 accommodates an engine 110, an intake manifold 112, an air cleaner 113, a radiator 114, a battery 102 and so on. FIG. 1 shows a 4-cylinder engine.
Each cylinder of the engine 110 is provided with a plug hole (not shown) in which an ignition plug is inserted. A mixture of air, which is introduced through the air cleaner 113 and the intake manifold 112, and a fuel from a fuel tank (not shown) is supplied into each cylinder of the engine 110. If the ignition plug is ignited (sparked) at an appropriated timing, the engine is started and rotated.
FIG. 2 is a block diagram of a portion of an electric system of a vehicle 100r. The electric system of the vehicle 100r includes a battery 102, an ignition coil 104, an ignition plug 106, an ECU 108 and an igniter 200r. The ECU 108 periodically generates an ignition signal IGT (Ignition Timing) indicating an ignition timing of the ignition plug 106 in synchronization with the rotation of the engine 110. A secondary coil L2 of the ignition coil 104 is connected to the ignition plug 106. The igniter 200r controls a current of a primary coil L1 of the ignition coil 104 in response to the ignition signal IGT. Accordingly, a high voltage (secondary voltage VS) of tens of kV can be generated in the secondary coil L2 to discharge the ignition plug 106 so that the mixture in the engine 110 is exploded.
The igniter 200r includes a switch device 202 and a switch controller 300r. The switch device 202 may be an IGBT (Insulated Gate Bipolar Transistor) having a collector connected to the primary coil L1 and an emitter grounded. In response to the ignition signal IGT, the switch controller 300r controls a voltage of a control terminal (gate) of the switch device 202 to control ON/OFF of the switch device 202. Specifically, the switch controller 300r puts the switch device 202 under an ON state in a period for which the ignition signal IGT has a high level. When the switch device 202 is turned on, a battery voltage VBAT is applied across the primary coil L1 and the current flowing through the primary coil L1 increases according to time passage. When the ignition signal IGT transitions to a low level, the switch controller 300r turns off the switch device 202 instantly to cut off the current IL1 of the primary coil L1. At this time, a primary voltage VL1 of hundreds of V (=L·dIL1/dt) that is proportional to the time derivative of the current IL1 is generated in the primary coil L1. At this time, the secondary voltage VS of tens of kV, which is a product of the primary voltage VL1 and a winding ratio, is generated in the secondary coil L2.
The switch controller 300r includes a front determination stage 300A and a rear drive stage 300B. The determination stage 300A receives the ignition signal IGT from the ECU 108 and determines its level (High/Low). Here, the igniter 200r used in the engine room is exposed to a variety of surge noises and high frequency noises. In order to prevent the igniter 200r from malfunctioning due to the high frequency noises, a high frequency filter 303 to eliminate the high frequency noises superimposed on the ignition signal IGT is provided in the determination stage 300A. A determination comparator 302 compares a voltage level VFIL of the ignition signal IGT passed through the high frequency filter 303 with a predetermined reference voltage (threshold value) VREF to generate a bi-leveled (High/Low) determination signal SDET based on a result of the comparison.
The drive stage 300B switches ON/OFF of the switch device 202 in response to the determination signal SDET. A delay circuit 304 gives a predetermined delay to the determination signal SDET. An amount of the delay is set such that a time lag (delay) between the transition of the ignition signal IGT and the discharging of the ignition plug becomes a predetermined value. A pre-driver 306 and a gate driver 308 control the gate voltage of the switch device 202 in response to an output of the delay circuit 304.
An ignition circuit (hereinafter referred to as an IGF (Ignition Feedback) circuit) 340r monitors a coil current IC flowing through the switch device 202 and informs the ECU 108 of an IGF signal indicating a result of the monitoring. Specifically, two reference currents ITHL and ITHH are defined in the IGF circuit 340r. If IC<ITHL or ITHH<IC, the IGF signal has a first level (e.g., high level). If ITHL<IC<ITHH, the IGF signal has a second level (e.g., low level).
FIG. 3 shows a circuit diagram of the IGF circuit 340r examined by the present inventor. The IGF circuit 340r includes comparators CMP1 and CMP2, a logic gate 342 and an output transistor 344. A resistor RCS for current detection is interposed between the emitter of the switch device 202 and ground. A voltage drop (detection voltage VCS) that is proportional to the coil current IC is generated in the resistor RCS.
The comparator CMP1 compares the detection voltage VCS with a reference voltage VTHH corresponding to the first reference current ITHH and outputs a comparison signal S11 having a high level when VTHH<VCS. The comparator CMP2 compares the detection voltage VCS with a reference voltage VTHL corresponding to the second reference current ITHL and outputs a comparison signal S12 having a high level when VTHL<VCS. The logic gate 342 performs a logic operation for the two comparison signals S11 and S12 to output a feedback signal S13. For example, the logic gate 342 produces a logical product of the comparison signal S12 and an inversion of the comparison signal S11.
The output transistor 344 of the IGF circuit 340r is of an open drain (open collector) type. The output transistor 344 has a drain connected to an IGF terminal, a source connected to a ground line 312, and a gate to which the output S13 of the logic gate 342 is input. The IGF terminal and the ECU 108 are interconnected via a harness 116. A signal line 118 of the harness 116 that transmits the IGF signal is pulled-up in the ECU 108. The ECU 108 detects whether or not the igniter 200r is normally operating, based on a voltage waveform (edge timing or pulse width) Vx of the signal line 118.
As a result of the examination on the igniter 200r of FIG. 3, the present inventor came to recognize the following problems.
Here, the igniter 200r used in the engine room is exposed to a variety of surges and noises (hereinafter, collectively referred to as noises). Therefore, the igniter 200r is increasingly required to have resistance to various noises.
FIG. 4 is an operation waveform diagram of the igniter 200r of FIG. 3. It is noted that vertical and horizontal axes of waveform diagrams or time charts referred to in the present disclosure are appropriately enlarged or reduced for ease of understandings. Further, the waveforms shown are simplified, exaggerated or emphasized for ease of understanding. If a surge or noise S20 is introduced in the igniter 200r, the detection voltage VCS, a potential VGND of the ground line 312 or a power supply voltage VDD may be varied. Thus, chattering occurs on the comparison signals S11 and S12 to propagate to the feedback signal S13. As a result, the IGF signal is varied.
In addition, due to a noise S21, timings of change in levels of the comparison signals S11 and S12 may be shifted, which may result in deviation in timings of the feedback signal S13 and further the IGF signal.
The variation (chattering) of the feedback signal S13 due to the chattering of the comparison signals S11 and S12 may be managed in some degrees by setting a hysteresis in the comparators CMP1 and CMP2.
On the other hand, the ground voltage VGND may be swung due to a noise introduced in the ground line 312. In addition, if a noise is introduced in the gate of the output transistor 344, even when the comparison signals S11 and S12 are operated normally, due to the noise, the output transistor 344 may be switched to cause the IGF signal to swing.